Parameter Encoding and Decoding

1. An audio synthesizer for generating a synthesis signal from a downmix signal comprising a number of downmix channels, the synthesis signal comprising a number of synthesis channels, the downmix signal being a downmixed version of an original signal comprising a number of original channels, the audio synthesizer comprising: a first path comprising: a first mixing matrix block configured for synthesizing a first component of the synthesis signal according to a first mixing matrix calculated from: a covariance matrix of the synthesis signal; and a covariance matrix of the downmix signal, a second path for synthesizing a second component of the synthesis signal, wherein the second component is a residual component, the second path comprising: a prototype signal block configured for upmixing the downmix signal from the number of downmix channels to the number of synthesis channels; a decorrelator configured for decorrelating the upmixed prototype signal; a second mixing matrix block configured for synthesizing the second component of the synthesis signal according to a second mixing matrix from the decorrelated version of the downmix signal, the second mixing matrix being a residual mixing matrix, wherein the audio synthesizer is configured to calculate the second mixing matrix from: the residual covariance matrix provided by the first mixing matrix block; and an estimate of the covariance matrix of the decorrelated prototype signals acquired from the covariance matrix of the downmix signal, wherein the audio synthesizer further comprises an adder block for summing the first component of the synthesis signal with the second component of the synthesis signal.

2. The audio synthesizer of claim 1, wherein the residual covariance matrix is acquired by subtracting, from the covariance matrix of the synthesis signal, a matrix acquired by applying the first mixing matrix to the covariance matrix of the downmix signal.

3. The audio synthesizer of claim 1, configured to define the second mixing matrix from: a second matrix which is acquired by decomposing the residual covariance matrix of the synthesis signal; a first matrix which is the inverse, or the regularized inverse, of a diagonal matrix acquired from the estimate of the covariance matrix of the decorrelated prototype signals.

4. The audio synthesizer of claim 3, wherein the diagonal matrix is acquired by applying the square root function to the main diagonal elements of the covariance matrix of the decorrelated prototype signals.

5. The audio synthesizer of claim 3, wherein the second matrix is acquired by singular value decomposition, SVD, applied to the residual covariance matrix of the synthesis signal.

6. The audio synthesizer of claim 3, configured to define the second mixing matrix by multiplication of the second matrix with the inverse, or the regularized inverse, of the diagonal matrix acquired from the estimate of the covariance matrix of the decorrelated prototype signals and a third matrix.

7. The audio synthesizer of claim 6, configured to acquire the third matrix by SVP applied to a matrix acquired from a normalized version of the covariance matrix of the decorrelated prototype signals, where the normalization is to the main diagonal the residual covariance matrix, and the diagonal matrix and the second matrix.

8. The audio synthesizer of claim 1, configured to define the first mixing matrix from a second matrix and the inverse, or regularized inverse, of a second matrix, wherein the second matrix is acquired by decomposing the covariance matrix of the downmix signal, and the second matrix is acquired by decomposing the reconstructed target covariance matrix of the downmix signal.

9. The audio synthesizer of claim 1, configured to estimate the covariance matrix of the decorrelated prototype signals from the diagonal entries of the matrix acquired from applying, to the covariance matrix of the downmix signal, the prototype rule used at the prototype block for upmixing the downmix signal from the number of downmix channels to the number of synthesis channels.

10. The audio synthesizer of claim 1, wherein the audio synthesizer is agnostic of the decoder.

11. The audio synthesizer of claim 1, wherein bands are aggregated with each other into groups of aggregated bands, wherein information on the groups of aggregated bands is provided in the side information of the bitstream, wherein the channel level and correlation information of the original signal is provided per each group of bands, so as to calculate the same at least one mixing matrix for different bands of the same aggregated group of bands.

12. A method for generating a synthesis signal from a downmix signal comprising a number of downmix channels, the synthesis signal comprising a number of synthesis channels, the downmix signal being a downmixed version of an original signal comprising a number of original channels, the method comprising the following phases: a first phase comprising: synthesizing a first component of the synthesis signal according to a first mixing matrix calculated from: a covariance matrix of the synthesis signal; and a covariance matrix of the downmix signal, a second phase for synthesizing a second component of the synthesis signal, wherein the second component is a residual component, the second phase comprising: a prototype signal step upmixing the downmix signal from the number of downmix channels to the number of synthesis channels; a decorrelator step decorrelating the upmixed prototype signal; a second mixing matrix step synthesizing the second component of the synthesis signal according to a second mixing matrix from the decorrelated version of the downmix signal, the second mixing matrix being a residual mixing matrix, wherein the method calculates the second mixing matrix from: the residual covariance matrix provided by the first mixing matrix step; and an estimate of the covariance matrix of the decorrelated prototype signals acquired from the covariance matrix of the downmix signal, wherein the method further comprises an adder step summing the first component of the synthesis signal with the second component of the synthesis signal, thereby acquiring the synthesis signal.

13. A non-transitory digital storage medium having a computer program stored thereon to perform the method for generating a synthesis signal from a downmix signal comprising a number of downmix channels, the synthesis signal comprising a number of synthesis channels, the downmix signal being a downmixed version of an original signal comprising a number of original channels, the method comprising the following phases: a first phase comprising: synthesizing a first component of the synthesis signal according to a first mixing matrix calculated from: a covariance matrix of the synthesis signal; and a covariance matrix of the downmix signal, a second phase for synthesizing a second component of the synthesis signal, wherein the second component is a residual component, the second phase comprising: a prototype signal step upmixing the downmix signal from the number of downmix channels to the number of synthesis channels; a decorrelator step decorrelating the upmixed prototype signal; a second mixing matrix step synthesizing the second component of the synthesis signal according to a second mixing matrix from the decorrelated version of the downmix signal, the second mixing matrix being a residual mixing matrix, wherein the method calculates the second mixing matrix from: the residual covariance matrix provided by the first mixing matrix step; and an estimate of the covariance matrix of the decorrelated prototype signals acquired from the covariance matrix of the downmix signal, wherein the method further comprises an adder step summing the first component of the synthesis signal with the second component of the synthesis signal, thereby acquiring the synthesis signal,, when said computer program is run by a computer.